1. Field of the Invention
The present invention is related to light sensing systems. More particularly, the present invention is related to a novel light and dark sensing system for thin paper having sprocket holes on the edges such as is presently used in high speed printers employed at the output of computers.
2. Description of the Prior Art
Apparatus for sensing the presence or absence of an aperture in a moving object are generally well known. IBM cards and Hollerith cards for looms have been read by various means including light sensing means before the advent of the electronic computer.
Prior art light sensing systems have employed light emitting diodes as a light source and photo transistors as light sensing devices. For purposes of this discussion, the prior art systems generally fall into two types. Either the light source is directed completely through the aperture in the object and sensed on the opposite side or the light source is directed to markings on the object and the amount of relfected light is employed to determine the presence or absence of marking on the object. In the latter type of system it is known that a high light to dark ratio is required to accurately detect marking.
High speed printers for computers have encountered similar problems. Rapid movement of the sprocketed paper generates a fine paper dust or powder which is highly reflective. This dust may collect in the apertures of the paper or in recesses of the paper guides, causing the light detecting devices to sense the absence of an aperture where, in fact, an aperture exists.
The paper presently employed in high speed printers for computers is translucent and very thin. It is possible to read printed copy through one sheet of such paper when sheets are placed one upon the other. Further, the translucent paper transmits a high percentage of light even though it is somewhat diffused.
There is no way for a light sensing device to determine whether the light rays it receives are diffused light passing through thin translucent paper or whether the diffused light is the result of reflections and/or blockages caused by the fine paper powder or dust generated in high speed printers.
It has been suggested that the light sources and the light sensing devices be sealed or encapsulated from vapor or dust. While this arrangement provides improved results it does not eliminate the paper dust or powder or the light diffusion caused by its presence in the system.
It would be extremely desirable to provide a hole sensing detector system which is cheap and reliable and operates in spite of adverse conditions presented by the presence of paper powder and dust.